According to conventional technique, a thermoplastic foam may be produced by feeding a thermoplastic resin into an extruder and, through the shearing action of one or more screws, melting the resin continuously in the barrel of the extruder. In an intermediate or a mixing section, a blowing agent, usually in a liquid or gaseous state, is continuously injected into the molten resin. In some instances, a chemical blowing agent may be dispersed throughout the particulate resin in a powder form before the resin is fed to the extruder as discussed in U.S. Pat. No. 4,107,260. In either case, the extruder screw is designed to mix and dissolve the blowing agent as uniformly as possible in the molten resin. Thorough, uniform mixing is essential to the production of a high quality foam. The resultant mixture must be maintained under carefully controlled temperatures and pressures within the extruder in order to prevent the volatilization of the blowing agent. When the molten mixture is forced through a die, the material undergoes decompression to atmospheric pressure so that the blowing agent separates within the body of material as bubbles. If the temperature is too high, there is overexpansion and the cells rupture. If the temperature is too low, there is incomplete expansion, resulting in a low quality foam. In many instances, the temperature window between overexpansion and underexpansion is only a few degrees Fahrenheit.
In many applications, it is desirable to use a low density foam. Among the areas where a low density foam proves useful are insulation and weatherstripping. In general, weatherstrips are used to seal joints or spaces between doors and windows to stop infiltration of air, rain, snow, and other elements. Effective weatherstripping serves to reduce heating costs in winter and cooling costs in summer. A weatherstrip must have certain characteristics to produce an effective seal. First, it should have a good resistance to compression set. Compression set resistance refers to the ability of a material to resume its initial shape after being subjected to a compressive load. Failure to do so may result in an uneven seal, reducing the effectiveness of the weatherstrip. Second, a weatherstrip must be soft and yielding, i.e. it must be easily compressible. This ensures that the door or window can be closed without the need for excessive force and still compress the weatherstrip sufficiently to form the necessary seal. Finally, it is desirable that a weatherstrip be lightweight, i.e. have a low density.
Some these properties have been achieved in prior art weatherstrip products by the use of polyurethane foam materials. Polyurethane foams, however, have a number of significant disadvantages in the manufacture of weaterstrip products. Most importantly, polyurethane foams are not thermoplastic, which results in the need for expensive molding techniques. These molding techniques cannot be easily adapted to products of different cross sections and any waste generated cannot be reused. Another disadvantage of polyurethane foams is their high percentage of open cells which results in undesirable uptake of water, e.g. rain. These water absorption properties deleteriously affect the performance of the polyurethane foam weatherstrip product in adverse weather conditions.
Thus, a low density thermoplastic foam which is easily compressible and has a good compression set resistance would serve as a good weatherstrip. To produce lightweight foams, a substantial percentage of blowing agent must be introduced into the molten resin. Because of the amount of blowing agent requird for a low density foam. thorough mixing and cooling of the molten mixture becomes an even more pronounced problem. In general, as the molten material passes through the extruder, the temperature increases due to the combined shear and compressive forces applied to the material by the rotating extruder screw. The magnitude of the temperature increase varies according to the rotation rate of the extruder screw and the shear properties of the resin being used.
One method to control the temperature increase would be to lower the screw speed, but this would result in a decreased production rate. U.S. Pat. No. 4,222,729 interposes a cooling/mixing device between the extrusion head and the barrel of the extruder. However, while such devices do increase cooling, there is still a problem of achieving a truly uniform temperature distribution. It is also common to employ a tandem extruder, i.e. a second extruder coupled to the primary extruder which serves to cool the molten mixture. The second extruder is generally larger and has a screw or screws rotating at a lower velocity than the primary extruder. This configuration suffers from several serious drawbacks. First, since the second extruder is independently driven, it requires separate drive and control mechanisms which can double the overall equipment costs. Second, it is necessary to provide a good seal at the point where the second extruder is driven. These seals are very expensive and frequently ineffective. It is therefore desirable to provide an extrusion apparatus which overcomes these difficulties.
In the Monsanto Company publication "Extrusion Foaming Technology for SANTOPRENE.RTM. Thermoplastic Rubber", a method is described for producing extrusions of foamed SANTOPRENE.RTM. with densities of foam 12-44 lbs/ft.sup.3. A tandem extruder system is used with the primary extruder employed for plasticating and for fluorocarbon addition. The downstream extruder, usually one size larger, functions mainly as a cooling and mixing extruder. High pressure crossover piping is used to connect the two machines. However, as noted above, a tandem extruder is generally more expensive and complicated to operate. Any low density foams that can be produced by the described method can only be produced when hard grades of the thermoplastic elastomeric material are used. Because of the increased stiffness and poorer compression set resistance of harder grades of the material, any resulting foams are unsuitable for use as a weatherstrip.